The animal embryo develops until it can survive on its own, at which point the egg hatches. Most vertebrates, arthropods and molluscs lay eggs outside the mother's body. They are always in some kind of container, a shell or covering.

Some animals, mostly mammals, keep the zygote inside their body where the embryo grows until it is ready to be born. These are also cleidoic eggs, but all the development is done inside the mother's body, as with humans.

Most mammals, and a number of marine reptiles, give live birth. Also, there are some invertebrates, such as scorpions, where the eggs develops inside the mother. They also give birth to live young.

The ostrich has the largest eggs of all living animals. Eggs, mostly bird eggs, are often eaten as food. The shell of an egg is a hard calcareous (CaCO3) material. The shell of an ostrich egg can support the weight of a fully grown human.